A. Field of the Invention
The present invention relates to the field of data communications, and more particularly, to the communication of facsimiles over data networks.
B. Background of the Invention
Wide-area data networks, the Internet in particular, have grown in their reach and capability to the point where they provide a practical alternative infrastructure for performing many communications functions that are presently performed over the general switched telephone network (GSTN). Voice telephone calls over a data network such as the Internet, referred to as Voice-over-IP (xe2x80x9cIPxe2x80x9d refers to xe2x80x9cInternet Protocolxe2x80x9d, a network protocol used for transporting data over the Internet), allows callers to converse over the telephone with only limited use of the GSTN equipment provided by the local and long distance service providers.
In comparison to dedicated circuit-switched connections established by the GSTN, the Internet provides voice communications, as well as multimedia communication such as text, graphics, video and audio, over a packet-based network. A voice over IP call establishes a virtual call connection between two or more callers through the system of interconnected packet-based networks that make up the Internet, intranets and other digital networks that provide connectivity between users.
In a virtual call connection, voice information is typically carried to a network gateway over a local GSTN connection as G.711-coded signal. The voice information is broken up into packets that are transmitted over the different networks that carry the virtual connection to a second network gateway near another party to the connection. The packets are converted back to G.711-coded signals, which are carried over a second local connection to the other party. By using the public Internet to carry the packets, users substantially avoid the fees and charges levied by the long distance service providers who provide the GSTN equipment and service. The VoIP system can be a desirable alternative to those telephone users who wish to save on their telephone bills and can tolerate the occasional delays and dropouts or loss of quality of service due to data traffic congestion on the public Internet.
Similar advantages are available to users who may wish to communicate facsimiles over the Internet. A virtual connection may be created for a facsimile transmission of one or more documents between callers by using facsimile machines at each end. The G.711-coded signals that carry the facsimile information may be packetized by the network gateways for transport over the public Internet in the same way as voice over IP calls.
One problem with using voice over IP virtual call connections for communicating facsimile information is that it makes inefficient use of the bandwidth. A voice over IP connection using G.711 has an average bandwidth requirement of 100 kbps and a typical facsimile connection has a data rate of 9600 bps. It would be inefficient to use a voice over IP virtual call connection for facsimile connections requiring only approximately {fraction (1/10)}th of its bandwidth. Furthermore, Voice over IP connections are fill-duplex, while 15 facsimile is half-duplex. Hence using a G.711 Voice over IP call to carry facsimile uses about 20 times more bandwidth than theoretically needed. It would be desirable to communicate a facsimile without using more bandwidth than that required by the connection rate of the facsimile transmission.
Another problem with using voice over IP connections for facsimile transmission is that they become susceptible to lost packets. Voice over IP connections are typically made using a connection-less, unreliable UDP protocol. However, voice connections are more tolerant of lost packets than facsimile transmissions. A lost packet in a voice connection may not be perceptible to the parties listening. Lost packets in facsimile transmissions may result in poor image reproduction, or at worst, an unsuccessful transmission.
One solution for making facsimile transmissions over the Internet, referred to as xe2x80x9cFax over IPxe2x80x9d, more efficient and more reliable is to use a store-and-forward technique.
In the store-and-forward technique, the sending facsimile machine connects to a facsimile receiver on the network gateway using T.30, the protocol used to control interaction between two facsimile machines. The facsimile receiver demodulates the facsimile signal and stores the document, or portions of the document, in memory as facsimile data before forwarding it over the Internet to another network gateway. When the network gateway receives the document or portions of the document, a facsimile sender on the other network gateway connects to the destination facsimile machine using the T.30 recommendation. The facsimile sender converts the facsimile data to the facsimile signal and sends the facsimile signal to the destination facsimile machine. The destination facsimile machine uses the facsimile signal to produce the hard copy document at the destination facsimile machine.
The facsimile receiver/sender may demodulate/modulate the facsimile signal according to standard modulation protocols, one of the most common ones being the V.21 modulation protocol. The V.21 modulation protocol uses a 300 bps data rate, but includes a digital command signal for negotiating to higher data rates (e.g. 9600 and 14,400). The V.21 modulation protocol may be used with the T.30 control procedures to provide an error correction mechanism, which permits transmission at a higher speed, but retransmissions at lower speeds when errors occur on noisy connections.
The network gateway includes a network interface that receives the demodulated facsimile signal from the facsimile receiver and packetizes the signal for transmission over the data network. The T.38 xe2x80x9cPROCEDURES FOR REAL TIME GROUP 3 FACSIMILE COMMUNICATION BETWEEN TERMINALS USING IP NETWORKSxe2x80x9d draft recommendation from the International Telecommunications Union (ITU) describes technical features for transporting the demodulated facsimile signal over the Internet.
One feature described in the T.38 recommendation is a payload format for organizing the packets of facsimile data. The facsimile data packets include a header for carrying transport information and a payload for carrying the data being transported.
One problem with using the store-and-forward method for Fax over IP is that it may not achieve xe2x80x9creal-timexe2x80x9d communication reliably. In the context of facsimile communication, xe2x80x9creal-timexe2x80x9d refers to the production of a document at the destination facsimile machine as the user inserts pages at the sending facsimile machine. By definition, the store-and-forward technique cannot transmit in real-time if the document is demodulated and stored in the first network gateway in its entirety before being sent over the network.
It may be possible to transmit in real-time if portions of the document are demodulated and sent over the network. However, the amount of processing required makes it difficult. Using the store-and-forward technique, the document is modulated at the transmitting facsimile machine, demodulated at the first network gateway, re-modulated at the second network gateway and demodulated once more at the destination facsimile machine. Each demodulation and modulation consumes time in processing that makes it more difficult to achieve real-time transmission.
It would be desirable to reliably transmit facsimiles over IP in real-time.
In view of the foregoing, a system is provided for communicating a facsimile in which first and second facsimile devices are connected to a general switched telephone network to send and receive a facsimile signal over the general switched telephone network. The facsimile signal is generated by encoding an image and sending the encoded image over the general switched telephone network at a modulation data rate. A transmitting gateway is connected to the general switched telephone network and to a data network. The transmitting gateway includes a facsimile receiver for receiving the facsimile signal. The facsimile receiver detects the facsimile signal by detecting the modulation data rate and converts the facsimile signal to a plurality of data units when the modulation data rate is detected.
The facsimile receiver also includes a transmitting network interface for formatting the data units as a data network signal. The transmitting network interface transmits the data network signal over the data network. The data network transports the data network signal to a receiving gateway, which includes a receiving network interface for receiving the data network signal and a facsimile sender. The facsimile sender converts the data network signal to a received facsimile signal and sends the received facsimile signal over the general switched telephone network to the second facsimile device.
In another aspect of the present invention, a method is provided for communicating a facsimile. A facsimile signal representing an encoded image is generated and transmitted over a general switched telephone network connection to a transmitting gateway at a modulation data rate. The facsimile signal is detected at the transmitting gateway. The facsimile signal is converted to a data network signal when the facsimile signal is received at the modulation data rate without decoding the image. The data network signal is transported over a data network to a receiving gateway. The data network signal is converted to a received facsimile signal and sent to a second facsimile device.